The thermoelectric properties of the all-carbon material graphene have been used to create a new kind of radiation detector, a rapid-response bolometer. The new device has fast response times and, unlike radiation detectors can function over a wide range of temperatures. The low-cost system could be readily scale for commercial applications the work of Grigory Skoblin of Chalmers University of Technology in Sweden and colleagues suggests. [Skoblin et al Appl Phys Lett, (2018) 112, 063501; DOI: 10.1063/1.5009629]
The much-heralded discovery in 2004 of the carbon allotrope, graphene, which exists as essentially a single atomic layer material offered new hope for many technologies of an alternative to more conventional materials. Its development has not quite panned out as the early hype would have suggested it might, according to Skoblin: "Unfortunately, there are some strong fundamental limitations for this material," he says. "The real industrial applications of graphene are quite limited."
However, there are niche applications that could exploit some of graphene's fundamental properties. In the team's graphene bolometer, radiation impinging on the device leads to heating which induces electrons to move. This generates an electric field and thus a potential difference across the device. This change in voltage gives a proxy measurement of the amount of radiation hitting the device. Other devices rely on an external power supply to reveal this voltage different. The simple mechanism manifest in the thermoelectric effect in the graphene device avoids the need for an external power supply. The piece of graphene used in the new bolometer is small, which makes it quick to respond to radiation. Its nature means it can continue operating up to a temperatures of around 200 degrees Celsius; conventional bolometers only operate at cryogenic temperatures, which is another significant limitation.
There have been other experimental bolometers made using graphene. However, these required a double layer of graphene, which makes them much harder to scale, Skoblin suggests. In an earlier device, the team found a relatively easy way to coat graphene with the dielectric polymer Parylene. However, they have now demonstrated that hexagonal boron nitride can be used as a performance-enhancing coating that is even easier to handle thatn Parylene. Other additional tests of efficacy are needed.
The team points out that their prototype bolometer operates only with microwave radiation at 94 gigahertz. They suggest that a future design will expand the range of radiation that might be detected.
David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".